The only reason for using a proxy is to restrict internet access and possibly provide a filter / virus scan on incoming traffic. This is the standard arrangement for corporate networks, but for a small office you can opt to go direct and live with everyone having full access to the internet. If you choose the direct option you need to ensure all machines, including the server, have up to date virus and malware protection - you should have these anyway - and that staff are versed in the rules of internet engagement, no porn etc.

Do you have email on the server and how does the server access the internet to send mail?
Do you want the proxy version and also the added cost, compexity and management, or are you happy to go direct?
Does your ISP router allow configuration / filtering?
Static IP addresses are not required for either method.

1) Yes, we have exchange accounts inorder to have intra communications.
2) I just knew, Proxy has too many factors to decide. Will opt for direct access!
3) The ISP has provided two DNS address', the clients are able to connect to the internet only when those address' are set.

imagine there's a network with intranet connectivity or LAN connectivity but no Internet connectivity, and later we decide to have a internet connectivity. We subscribe for a ISP and ISP gives us a leased line connectivity and a Router.

In this scenario, what am I suppose to do? How should I connect the ISP's router so that the whole network gets internet connectivity?

As you already have a domain you can use the existing DNS to send all non-domain traffic via the router.
You need the router plugged into the switch so all machines can access it. Is this your current set up?
I assume you have a DHCP server on your Windows server?
Configure DNS forwarding on the AD DNS server for all non-domain requests. Use the 2 ISP DNS servers as the forwarded addresses.
Now any request for an external IP will resolve and the router will NAT the traffic and send it over the internet.
Note: the default gateway set on the DHCP server will need to be the local IP address of the router.

I'm not quite sure what your ISP means by "wan ip", but the "usable range" provides public IP addresses (on the internet) for you to use for things like your mail server or public web site. They have nothing to do with static IP addresses on your client machines - you should never use static IP addresses on your client machines, that is what the DHCP server is for.

Do you currently have a public facing mail server or is it only internal?
Do you have a public domain name?
Is your internal AD name the same as your (intended) public domain name (this is bad, m'kay).

WAN is usually a VPN provided by your ISP to allow you to network multiple sites. This IP address is not accessible from the internet.
LAN would be internet connected IP addresses which you would use for email, web, VPN endpoints etc.
All you need to do is connect the link to your firewall / router and set up the NAT entries etc.